It is often desirable to determine the location of a wooden member in a wall so that a fastener, such as a screw or nail, can be driven into the wall and, especially, directly into the member to support a shelf, picture or other article which might be too heavy to be supportable by the wall area between such members.
Vertical support members, usually called studs, can be approximately located by using magnetic devices which respond to the presence of the nails normally used to fasten materials such as gypsum wallboard or panelling to the studs. Unfortunately, the nails are not known to be at or very near to the stud center so that, even if one finds a nail magnetically, it is quite difficult to tell how far the stud center is from that nail and in which direction. Also, it is often rather difficult to determine the location of the nail accurately through the tape, spackle and paint which is used to finish the wallboard and seams.
It has therefore been determined that the most reliable and accurate technique for locating a stud involves putting something through a test hole in the wallboard which is capable of "feeling" the stud and which involves some sort of measuring technique. Examples of such devices, used in locating studs and various other items which are concealed from view, are found in the following U.S. patents.
U.S. Pat. No. 745,357, Lanham; PA1 U.S. Pat. No. 1,718,003, Prsha; PA1 U.S. Pat. No. 1,811,945, Lange; PA1 U.S. Pat. No. 2,480,725, Gilbert; PA1 U.S. Pat. No. 2,600,722, Atkinson; PA1 U.S. Pat. No. 2,992,488, Berrett; PA1 U.S. Pat. No. 3,048,925, Klaus.
Unfortunately, these devices generally require making a test hole which is rather large, an obviously undesirable requirement, or else they are not particularly accurate.